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Global-scale 21st-century glacier mass change projections from six published global glacier models are systematically compared as part of the Glacier Model Intercomparison Project. In total 214 projections of annual glacier mass and area forced by 25 General Circulation Models (GCMs) and four Representative Concentration Pathways (RCP) emission scenarios and aggregated into 19 glacier regions are considered. Global mass loss of all glaciers (outside the Antarctic and Greenland ice sheets) by 2100 relative to 2015 averaged over all model runs varies from 18 +/- 7% (RCP2.6) to 36 +/- 11% (RCP8.5) corresponding to 94 +/- 25 and 200 +/- 44 mm sea-level equivalent (SLE), respectively. Regional relative mass changes by 2100 correlate linearly with relative area changes. For RCP8.5 three models project global rates of mass loss (multi-GCM means) of >3 mm SLE per year towards the end of the century. Projections vary considerably between regions, and also among the glacier models. Global glacier mass changes per degree global air temperature rise tend to increase with more pronounced warming indicating that mass-balance sensitivities to temperature change are not constant. Differences in glacier mass projections among the models are attributed to differences in model physics, calibration and downscaling procedures, initial ice volumes and varying ensembles of forcing GCMs.

Worldwide glacier retreat and associated future runoff changes raise major concerns over the sustainability of global water resources(1-4), but global-scale assessments of glacier decline and the resulting hydrological consequences are scarce(5,6). Here we compute global glacier runoff changes for 56 large-scale glacierized drainage basins to 2100 and analyse the glacial impact on streamflow. In roughly half of the investigated basins, the modelled annual glacier runoff continues to rise until a maximum ('peak water') is reached, beyond which runoff steadily declines. In the remaining basins, this tipping point has already been passed. Peak water occurs later in basins with larger glaciers and higher ice-cover fractions. Typically, future glacier runoff increases in early summer but decreases in late summer. Although most of the 56 basins have less than 2% ice coverage, by 2100 one-third of them might experience runoff decreases greater than 10% due to glacier mass loss in at least one month of the melt season, with the largest reductions in central Asia and the Andes. We conclude that, even in large-scale basins with minimal ice-cover fraction, the downstream hydrological effects of continued glacier wastage can be substantial, but the magnitudes vary greatly among basins and throughout the melt season.

Assessing and modeling precipitation in mountainous areas remains a major challenge in glacier mass balance modeling. Observations are typically scarce and reanalysis data and similar climate products are too coarse to accurately capture orographic effects. Here we use the linear theory of orographic precipitation model (LT model) to downscale winter precipitation from the Weather Research and Forecasting Model (WRF) over the Juneau Icefield, one of the largest ice masses in North America (>4,000 km(2)), for the period 1979-2013. The LT model is physically-based yet computationally efficient, combining airflow dynamics and simple cloudmicrophysics. The resulting 1 kmresolution precipitation fields show substantially reduced precipitation on the northeastern portion of the icefield compared to the southwestern side, a pattern that is not well captured in the coarse resolution (20 km) WRF data. Net snow accumulation derived from the LT model precipitation agrees well with point observations across the icefield. To investigate the robustness of the LT model results, we perform a series of sensitivity experiments varying hydrometeor fall speeds, the horizontal resolution of the underlying grid, and the source of the meteorological forcing data. The resulting normalized spatial precipitation pattern is similar for all sensitivity experiments, but local precipitation amounts vary strongly, with greatest sensitivity to variations in snow fall speed. Results indicate that the LT model has great potential to provide improved spatial patterns of winter precipitation for glaciermass balance modeling purposes in complex terrain, but ground observations are necessary to constrain model parameters to match total amounts.

Glaciers spanning large altitudinal ranges often experience different climatic regimes with elevation, creating challenges in acquiring mass-balance and climate observations that represent the entire glacier. We use mixed methods to reconstruct the 1991-2014 mass balance of the Kahiltna Glacier in Alaska, a large (503 km(2)) glacier with one of the greatest elevation ranges globally (264-6108m a. s.l.). We calibrate an enhanced temperature index model to glacier-wide mass balances from repeat laser altimetry and point observations, finding a mean net mass-balance rate of -0.74 mw.e. a(-1)(+/-sigma = 0.04, std dev. of the best-performing model simulations). Results are validated against mass changes from NASA's Gravity Recovery and Climate Experiment (GRACE) satellites, a novel approach at the individual glacier scale. Correlation is strong between the detrended model-and GRACE-derived mass change time series (R-2 = 0.58 and p << 0.001), and between summer (R-2 = 0.69 and p = 0.003) and annual (R-2 = 0.63 and p = 0.006) balances, lending greater confidence to our modeling results. We find poor correlation, however, between modeled glacier-wide balances and recent single-stake monitoring. Finally, we make recommendations for monitoring glaciers with extreme altitudinal ranges, including characterizing precipitation via snow radar profiling.

Estimating the long-term mass balance of the high-Arctic Svalbard archipelago is difficult due to the incomplete geodetic and direct glaciological measurements, both in space and time. To close these gaps, we use a coupled surface energy balance and snow pack model to analyse the mass changes of all Svalbard glaciers for the period 1957-2014. The model is forced by ERA-40 and ERA-Interim reanalysis data, downscaled to 1 km resolution. The model is validated using snow/firn temperature and density measurements, mass balance from stakes and ice cores, meteorological measurements, snow depths from radar profiles and remotely sensed surface albedo and skin temperatures. Overall model performance is good, but it varies regionally. Over the entire period the model yields a climatic mass balance of 8.2 cm w.e.yr(-1), which corresponds to a mass input of 175 Gt. Climatic mass balance has a linear trend of -1.4 +/- 0.4 cmw.e.yr(-2) with a shift from a positive to a negative regime around 1980. Modelled mass balance exhibits large interannual variability, which is controlled by summer temperatures and further amplified by the albedo feedback. For the recent period 2004-2013 climatic mass balance was 21 cmw.e.yr(-)1, and accounting for frontal ablation estimated by Blaszczyk et al. (2009) yields a total Svalbard mass balance of -39 cmw.e.yr(-1) for this 10-year period. In terms of eustatic sea level, this corresponds to a rise of 0.037 mm yr(-1). Refreezing of water in snow and firn is substantial at 22 cm w.e.yr(-1) or 26% of total annual accumulation. However, as warming leads to reduced firn area over the period, refreezing decreases both absolutely and relative to the total accumulation. Negative mass balance and elevated equilibrium line altitudes (ELAs) resulted in massive reduction of the thick (> 2 m) firn extent and an increase in the superimposed ice, thin (< 2 m) firn and bare ice extents. Atmospheric warming also leads to a marked change in the thermal regime, with cooling of the glacier mid-elevation and warming in the ablation zone and upper firn areas. On the long-term, by removing the thermal barrier, this warming has implications for the vertical transfer of surface meltwater through the glacier and down to the base, influencing basal hydrology, sliding and thereby overall glacier motion.

The Susitna River draining from the highly glacierized Central Alaska Range has repeatedly been considered a potential hydro-power source in recent decades, raising questions about the effect of glacier changes on the basin's river runoff. We determine changes in the glacier area (1951-2010), elevation (1951-2010, 1951-2005 and 2005-2010), equilibrium line altitude (ELA, 1999-2015), and accumulation area ratio (AAR, 1999-2015) of the basin's five largest glaciers covering 587 km(2) (2010). We use the Landsat time series, as well as digital elevation models (DEMs) from 1951 (United States Geological Survey (USGS) aerial imagery), 2005 (Advanced Spaceborne Thermal Emission and Reflection Radiometer, ASTER), and 2010 (airborne interferometric synthetic aperture radar, IfSAR). The glaciers lost an area of 128 +/- 15 km(2) (16%) between 1951 and 2010. The mean ELA was located at 1745 +/- 88 m a.s.l. during 1999-2015. The glacier's annual ELAs do not show any significant trends. We found a glacier-wide elevation change of 0.41 +/- 0.07 m yr(-1) for the period 1951-2005 and -1.20 +/- 0.25 m yr(-1) for 2005-2010. The results indicate that the glaciers are in a state of retreat and thinning, and have been losing mass at an accelerated rate in recent years. The interpretation of the thickness changes is complicated by the glaciers' surge cycles.

Surge-type Black Rapids Glacier, Alaska, has undergone strong retreat since it last surged in 1936-1937. To assess its evolution during the late Twentieth and Twenty-first centuries and determine potential implications for surge likelihood, we run a simplified glacier model over the periods 1980-2015 (hindcasting) and 2015-2100 (forecasting). The model is forced by daily temperature and precipitation fields, with downscaled reanalysis data used for the hindcasting. A constant climate scenario and an RCP 8.5 scenario based on the GFDL-CM3 climate model are employed for the forecasting. Debris evolution is accounted for by a debris layer time series derived from satellite imagery (hindcasting) and a parametrized debris evolution model (forecasting). A retreat model accounts for the evolution of the glacier geometry. Model calibration, validation and parametrization rely on an extensive set of in situ and remotely sensed observations. To explore uncertainties in our projections, we run the glacier model in a Monte Carlo fashion, varying key model parameters and input data within plausible ranges. Our results for the hindcasting period indicate a negative mass balance trend, caused by atmospheric warming in the summer, precipitation decrease in the winter and surface elevation lowering (climate-elevation feedback), which exceed the moderating effects from increasing debris cover and glacier retreat. Without the 2002 rockslide deposits on Black Rapids' lower reaches, the mass balances would be more negative, by similar to 20% between the 2003 and 2015 mass-balance years. Despite its retreat, Black Rapids Glacier is substantially out of balance with the current climate. By 2100, similar to 8% of Black Rapids' 1980 area are projected to vanish under the constant climate scenario and similar to 73% under the RCP 8.5 scenario. For both scenarios, the remaining glacier portions are out of balance, suggesting continued retreat after 2100. Due to mass starvation, a surge in the Twenty-first century is unlikely. The projected retreat will affect the glacier's runoff and change the landscape in the Black Rapids area markedly.

We determine the geodetic mass balance of surge-type Black Rapids Glacier, Alaska, for the time periods 1980-2001 and 2001-2010 by combining modern interferometric synthetic aperture radar (InSAR)-derived digital elevation models (DEMs), DEMs derived from archival aerial imagery, laser altimetry data, and in situ surface elevation measurements. Our analysis accounts for both the large rockslides and terrain displacements caused by the 2002 M7.9 earthquake on the Denali fault, which runs through Black Rapids Glacier. To estimate uncertainties, we apply Monte Carlo simulations. For the earthquake-triggered rockslides we find a volume of 56.62 +/- 2.86 x 10(6) m(3), equivalent to an average debris thickness of 4.44 +/- 0.24 m across the 11.7 km(2) deposit area on the glacier. Terrain displacement due to the earthquake corresponds to an apparent glacier volume change of -53.1 x 106 m(3), which would cause an apparent specific mass balance of -0.19 meter water equivalent (mwe) if not taken into account. The geodetic mass balance of Black Rapids Glacier is -0.48 +/- 0.07 mwe a(-1) for the entire 30 year period, but more negative for the period 2001-2010 (-0.64 +/- 0.11 mwe a(-1)) than the period 1980-2001 (-0.42 +/- 0.11 mwe a(-1)), in agreement with trends indicated by in situ mass balance measurements. Elevation data indicate no net thickening of the surge reservoir between 1980 and 2010, in contrast to what is expected during the quiescent phase. A surge of Black Rapids Glacier in the near future is thus considered unlikely.

The anticipated retreat of glaciers around the globe will pose far-reaching challenges to the management of fresh water resources and significantly contribute to sea-level rise within the coming decades. Here, we present a new model for calculating the twenty-first century mass changes of all glaciers on Earth outside the ice sheets. The Global Glacier Evolution Model (GloGEM) includes mass loss due to frontal ablation at marine-terminating glacier fronts and accounts for glacier advance/retreat and surface elevation changes. Simulations are driven with monthly near-surface air temperature and precipitation from 14 Global Circulation Models forced by RCP2.6, RCP4.5, and RCP8.5 emission scenarios. Depending on the scenario, the model yields a global glacier volume loss of 25–48% between 2010 and 2100. For calculating glacier contribution to sea-level rise, we account for ice located below sea-level presently displacing ocean water. This effect reduces the glacier contribution by 11–14%, so that our model predicts a sea-level equivalent (multi-model mean ±1 standard deviation) of 79±24 mm (RCP2.6), 108±28 mm (RCP4.5), and 157±31 mm (RCP8.5). Mass losses by frontal ablation account for 10% of total ablation globally, and up to ∼30% regionally. Regional equilibrium line altitudes are projected to rise by ∼100–800 m until 2100, but the effect on ice wastage depends on initial glacier hypsometries.